Brassia Maculata with Browning Pseudobulbs

[WaterWitchin]

Rot. Likely either broken down medium (brassia don't tolerate it well) or overwatering (brassia watering should be decreased a bit in winter) or a combination of both.

Were it mine, I'd unpot and examine media and roots, trim up and repot. How many "good" pseudobulbs (not rotted) does it have? I'd be removing the ones with rot. At least the ones that are as rotted as the main one you're focusing on in the pictures. Then err on the side of underwatering for a bit until you see it recovering, it gets warmer, etc. High humidity in the meanwhile.